Saturation Superheat Subcooling
To understand superheat or subcooling we must first understand Saturation
#1 Pressure (PSIG) converted to a Saturated temperature.
Suction saturated is the Blue, low pressure, suction gauge pressure converted to a vapor/ Dewpoint/ suction saturated temperature.
Liquid Saturated is the Red, high pressure, head pressure, liquid gauge converted to a liquid/ Bubblepoint/ head saturated temperature.
We convert the pressure to the saturated temperature of the refrigerant with a Pressure Temperature (TP) chart (aka saturated PT chart), an app or an automatic conversion wit digital guages.
#2 Liquid and Vapor both exist together.
In a tank of refrigerant, as long as there is any amount of liquid and vapor, it will follow the saturated TP chart.
In the evaporator, immediately after the metering devices unstill all the liquid is gone it is a saturated mixture. Ideally this will be most of the evaporator coil before all the liquid boils away.
In the condenser, the middle of the condenser starting approximately 2 rows from the top to approximately 2 rows form the bottom the refrigerant will be saturated, liquid vapor mixture.
#3 Change of state, Latent heat
Most heat is absorbed or rejected as the refrigerant changes state from a liquid to a vapor or from a vapor to a liquid, without changing temperature (latent heat). A massive amount of heat is absorbed or rejected as it changes state but the temperature stays the same as long as there is liquid and vapor. this is also referred to as hidden heat because the temperature does not increase even though its absorbing or rejecting heat.
In the evaporator, the refrigerant changes state from a liquid to a vapor absorbing a massive amount of heat (BTU) from the air even thought the temperatures is the same. This is often referred to as evaporator temperature aka evaporator saturated temperature. It will be this temperature until all of the liquid has changed state to a vapor.
In the condenser, where the refrigerant is changing state from a vapor into a liquid. This is where the refrigerant is rejecting the most amount of heat back into the air outside. It will be this temperature until all the vapor has changed state into a liquid.
#4 Condensing, Boiling/Evaporating
The condenser is where the refrigerant changes from a vapor to a liquid, rejecting a massive amount of heat into the cooler outdoor air. The condensing pressure and saturated temperature will be hotter than the outdoor air. As the refrigerant gives up heat to the cooler air, more of the refrigerant changes state to a liquid at the same temperature (latent). Condending temperature and liquid saturated temperature can be used interchangeably in this application.
The evaporator is where the refrigerant is violently boiling as the refrigerant changes state from a liquid to a vapor. the low pressure and Heat from the air makes the refrigerant boil, as the refrigerant boils it absorbs heat from the air. As the air gives up its heat to the refrigerant, more of the refrigerant changes state to a vapor.
Boiling is a cooling effect.
In our lives we have associated boiling to be hot because water boiling on a stove is hot to our bodies. However according to the saturated temperature of water at 0 PSIG it remains at a cool 212F untill all the water is gone. nomatter how much heat you apply to the water it stays at 212. If you where cooking roman, it would stay at a cool 212 until all the water was gone, the the noodles would increase in heat until it burned. By droping the pressure of water we can drop the boiling temperature and make water boil at room temperature. Every substance has a differentl boiling temperatures. Oxygen at 0 PSIG boils at -297F (saturated temp) and nitrogen boils at -320F (saturated temp). If you where to dip your hand in either of those boiling substances your hand would be frozen as the heat leaves your hand and moves into boiling nitrogen or oxygen. Boiling is a cooling effect.
R 410A boils at -60F at 0PSIG. if you where to tunr the tank upside down , the refrigerant would be boiling , changing state at -60F absorbing heat from antying warmer than it. If this was your hand you would have sever frostbite. If the evaporator pressure was at 119PSIG the saturated/Boiling temperature would be 40F. The refrigerant under that pressure is boiling changing state at 40F, absorbing heat from the cooler 75F air traveling on the outside of the evaporator coil the Warmer air makes the refrigerant boil and change state. the refrigerant boiling absorbs heat out of the air, cooling the air.
The refrigerant is boiling in the evaporator but since the term boiler was already used in plumbing the term Evaporator was used. Evaporation is also the change of state from liquid to vapor but generally in as a slower pace. This sometimes cause's confusion but we can use Evaporator temperature, Boiling Temperature, Suction Saturated temperature interchangeably.
Refrigeration equipment usually will not have a subcooling if there is a liquid receiver. The liquid receiver makes a liquid seal providing a full column of liquid to the metering device. Since the receiver is liquid and vapor in 1 container, you could be at 1 or even 0 subcooling but still be completely in liquid form. A sight glass will help you see into the system to ensure there are no vapor bubbles.
Don't add refrigerant bases on sight glasses alone. During a hot startup, the Txv could let all the refrigerant into the evaporator coil, so its best to let the system pull the box temperature down first. It is possible to check the subcooling leaving the condenser, before the liquid receiver.
Some refrigeration systems have a special subcooler added to increase the subcooling before the metering device.